Fork-lift trucks commonly comprise a load-carrying portion and a driving portion. The load-carrying portion has a mast which can be composed of several mast sections and which can be extracted to large heights, in case of need. A load-carrying means is movable in height on the mast by means of a lifting and lowering drive. The load-carrying means is primarily comprised of a so-called load-carrying fork which receives one or more pallets. A special type of fork-lift trucks is the so-called fork-lift reach truck the mast of which is horizontally movable between a position close to the driving section and a position remote therefrom. In addition, the load-carrying fork is movable transversely to the mast, mostly by means of an appropriate slider. A so-called side shift allows to orient the pallet precisely and quickly in the rack or the load-carrying fork to the pallet with no need for the fork-lift truck to change its position.
It is further known to vary the inclination of the load-carrying means and, hence, that of the fork. One option is to vary the mast inclination, e.g. to pivot the mast towards the driving portion to compensate the deflection of the mast, for example. Besides, a picked-up load will be supported more reliably during a travel mode if the load-carrying fork has an inclination by which the load has a tendency to slip towards the mast. The load-carrying fork which is to slide into a pallet requires that the load-carrying fork be substantially oriented horizontally. If the pallets exhibit an inclination from the horizontal line care should be taken to orient the fork correctly. Although the fork inclination can be varied by means of the inclination drive the driver of the fork-lift truck is not informed about the actual fork inclination. Specifically at large lifting heights, he cannot perceive whether the fork is oriented horizontally and can be smoothly slid into the pallet. One or more unsuccessful attempts to detect a pallet at a large height will naturally result in a longer handling time.
From DE 32 11 509 A1, it has become known to preset and store a target angle for the backward inclination of the mast in an industrial truck having a mast. The load resting on the fork is detected while modifying the backward inclination and the target angle. A sensor detects the real mast inclination relative to a vertical line in a no-load condition. The mast inclination angle detected is compared to the target angle stored. If the mast inclination angle detected is smaller than the target inclination angle an appropriate correction is made to the inclination angle. The arrangement described aims at obtaining a horizontal orientation of the prongs of the load-carrying fork, irrespective of which load is on the fork at which height. Floor irregularities, worn wheels, and age-induced deformations of the lift frame cannot be taken into account for the system described.
It is the object of the invention to provide a fork-lift truck in which the pallets can be picked up and stacked at larger lifting heights in a simple way.